According to Kahrun, the key ingredient for making the Lumaniciel weapon is Siel's Petal. When you visited Ekios, he gave you a Siel's Petal Bloom and asked you not to waste this valuable item. When you handed Siel's Petal, which you obtained by properly tending to the Bloom, to Ekios, he promised that he would send it to Siel's Temple.